docs: def add_proxy_auth(possible_proxy_url, proxy_auth):
      """
      Add a username and password to a proxy URL, if the input value is a proxy URL.
  
      :param str possible_proxy_url: Proxy URL or ``DIRECT``.
      :param requests.auth.HTTPProxyAuth proxy_auth: Proxy authentication info.
      :returns: Proxy URL with auth info added, or ``DIRECT``.
      :rtype: str
      """
      if possible_proxy_url == 'DIRECT':
          return possible_proxy_url
      parsed = urlparse(possible_proxy_url)
      return '{0}://{1}:{2}@{3}'.format(parsed.scheme, proxy_auth.username, proxy_auth.password, parsed.netloc)